Pope Benedict XVI's resignation explained

Vatican spokesman the Rev. Federico Lombardi said Pope Benedict had the right and potentially the duty to resign.

But he stressed that the pope's decision was not because of any external pressure.

Lombardi told reporters the pope was not unwell.

"There isn't any disease specifically -- it's all to do with deterioration and weakness in his body, as the holy pontiff has said in his statement," he said.

Lombardi said the battery in the pope's pacemaker -- which he has had since he was a cardinal -- had been replaced in the last few months, but he stressed that it was a routine procedure.

The pope said that he would step down at 8:00 p.m. on February 28, Rome time.

Lombardi said as far has he knew, the day was "just an ordinary day" without particular significance but that Pope Benedict might have chosen it to ensure a new pope was in place for Easter.

He said he understood the pope usually finished his working day at 8:00 p.m. which was why he had chosen that specific hour to step down.

It is the first time a pope has resigned in nearly 600 years.

The last pope to resign was Pope Gregory XII in 1415. He stepped down to end the "Great Western Schism" -- during which there were rival claims to the papal throne.

In 1294, Pope Celestine V resigned after only five months, preferring the simple life of a monk to the majesty of being pope.

According to the National Catholic Weekly, modern popes have felt that resignation is unacceptable and could encourage factions within the church to pressure pontiffs to step down.

Canon law states that a resignation must be made freely and properly manifested and that the pope resigning must be of sound mind.

A meeting of the College of Cardinals to elect the next pope will be held. The gathering is known as a "conclave."

Vatican spokesman Federico Lombardi said there would be elections some time in March and he anticipated that there would be a new pope before Easter.

Lombardi said Vatican scholars were studying the constitution to determine exactly when the conclave would begin.

By law, the conclave begins in the Sistine Chapel. On that morning, the cardinals will celebrate the Votive Mass, "Pro Eligendo Papa." That afternoon, the cardinals begin the election process.

The cardinals draw lots to select three members to collect ballots from the infirm, three "tellers" to count the votes and three others to review the results.

Blank ballots are then prepared and distributed. The ballots are rectangular in shape and must bear in the upper half, in print if possible, the words "Eligo in Summum Pontificem," meaning "I elect as supreme pontiff." The electors write the name of the candidate on the lower half and fold it in two.

After writing the name of one man on his ballot, each of the active cardinals -- those under 80 years of age -- walks to an altar in order of seniority and pledges to perform his duty with integrity. Each cardinal then places a folded ballot containing his choice onto a small disc made of precious metal and drop it inside a chalice.

After all votes are cast, the tellers tally the ballots and the result is read to the cardinals. If a cardinal receives two-thirds plus one of the votes, he is the new pontiff.

If there is no winner, another vote is taken. If there is still no winner, two more votes are scheduled for the afternoon.

After the votes are counted each time, the ballots are burned. If there has been no winner, a chemical is mixed with the ballots to produce black smoke when they are burned.

Sight of the black smoke emerging from the roof of the Vatican Palace tells those waiting in St. Peter's Square that a pope has not yet been selected.

When a winner has been selected, the ballots are burned alone, and the white smoke indicates there is a new pope.

Lombardi said the pope's appointments up until the end of February -- including audiences with the presidents of Romania and Guatemala -- had been confirmed.

On February 27, the pope's last general audience will take place. Lombardi said the Vatican was trying to arrange for it to be held in St Peter's square so that more people could attend and "to make it a sort of tribute to the holy pontiff."

Lombardi said it was expected that Pope Benedict would devote his time to reflection and prayer.

In his statement, Benedict said he wished "to also devotedly serve the Holy Church of God in the future through a life dedicated to prayer."

Lombardi said he didn't expect the pope to be given any other duty, given his age, and he said Benedict would no longer be involved in decision making or administrative duties.

Pope Benedict was "very discreet" and was "not someone who will interfere" or create issues in terms of his successor, he said.

After he steps down, Benedict will move to the pope's summer residence, Castel Gandolfo.

At a later date he will move into in a monastery at the Vatican.

The monastery has recently been renovated and extended to include a small chapel.

According to the Catholic Bishops' Conference of England and Wales, Pope Benedict will return to being known as Cardinal Joseph Ratzinger. It said he would be addressed as his eminence or Cardinal Ratzinger.

However, Lombardi later told reporters it was hard to imagine that the pope would be called a cardinal.